The term "seal" as employed herein refers not only to the action of blocking a flow path or path of leakage but also to various sealing devices or structures such as the resilient front ram packers of ram type blowout preventers and the annular sealing packer of an annular blowout preventer. From the front ram packers of ram type blowout preventers and the annular sealing packer of an annular blowout preventer. From the standpoint of valves the term "seal" is also employed in reference to various sealing elements such as seal rings, seat rings and the like.
Blowout preventers (BOP) are currently utilized during almost all oil well applications for the purpose of controlling well pressures in the event of a hydraulic imbalance that occurs when well pressures exceed the hydraulic hydrostatic pressures generated by the drilling fluid at formation level. Typical blowout preventers in use at this time normally consist of a BOP stack including four hydraulic rams and one or two "annular" blowout preventers which are activated by hydraulic fluid pressure from a hydraulic accumulator and pump system and function to close around the drill pipe or close the well bore when there is no pipe in the well. Special rubber devices (front ram packers) are provided on the ends of the rams either to close around the drill pipe (if the drill pipe is in the well bore), or to close off or seal the pipe after it has been sheared such as for an emergency disconnect procedure. The blowout preventer constitutes a valve device which seals the well bore after the drill pipe has been cut. The annular blowout preventers are provided for the purpose of closing around a wider range of pipe sizes or irregular shaped components of the drill string as well as closing on an open hole.
Since the BOPs are absolutely essential in controlling otherwise uncontrollable well bore pressures and are the final link in preventing catastrophic results in the event of a potential blowout, and because of other potentially dangerous and expensive possibilities, it is highly desirable to know the exact position of each ram of the blowout preventer at any point in time. Also, the relative position of the annular preventer closing mechanism is of importance in gauging seal wear and thus properly scheduling repair activities so as to save considerable repair costs including expensive service vessel costs for subsea repair activities.
Systems which have been experimented with in the past involve measuring the amount of hydraulic fluid pumped into the hydraulic cylinder and piston assembly to determine the ram position. However, such systems were subject to the inherent problem associated with hydraulic leaks and fluid compressibility. (The blowout preventer may be located at the bottom of the sea bed in as much as 12,000 feet of water). In this hostile environment it is appropriate to provide a system for absolutely establishing accurate determination of the positions of the various rams of the BOP so that well controlling activities may be efficiently accomplished. Further, the rams of BOPs must be cycled periodically to insure that they will be functional in the event of an emergency. During such cycling, seal wear will occur. It is also desirable therefore to provide an efficient means for accomplishing accurate determination of seal wear in order that the BOP may be restored to proper operating condition in the event wear of the ram seals has occurred.